Probably will have to open it up and look, maybe post a few pix of the conversion? It may be straitforward?
The switch would have input from the wheels and skates, then positions of off, smoke and lights, and run, and wiring to those functions.
My "bet" is the feed before the switch was cut, and this fed to the decoder. Then find the feed to the motor and accessories, simple chace down with a VOM and looking at the boards and switch.
The big question will be if the 5v lighting and smoke and board was neutered out of it?